The electrophysiological characteristics of canine atrial plateau fibers are being studied by the use of glass microelectrodes. These fibers may comprise the posterior internodal tract in the right atrium. The physiological role of such fibers is presently unknown. Current experiments are concerned with study of the effects of various agents and procedures on diastolic depolarization of such fibers. The effect of hyperthyroidism in such fibers also is to be studied.